A flat plate-shaped filament (also called as a “flat plate-shaped emitter”) including four legs for electrical heating will be described as an example of a filament including a plurality of electric flow paths for electric flow. Typical structures of the flat plate-shaped filament will be described with reference to FIGS. 6 and 7. FIGS. 6 and 7 are schematic plan views of the typical flat plate-shaped filaments. FIG. 6 is the flat plate-shaped filament having a rectangular shape, and FIG. 7 is the flat plate-shaped filament having a circular shape.
As illustrated in FIGS. 6 and 7, four legs 102 to 105 for electrical heating are provided at ends of an electron beam emission surface 101 (an electron beam emission surface 101 having a rectangular shape in FIG. 6, and an electron beam emission surface 101 having a circular shape in FIG. 7). Typically, electric flow is made through each of the legs 102 to 105 bent at 90° at the positions indicated by dashed lines in the figures, thereby heating the electron beam emission surface 101. Then, thermal electrons are emitted from the electron beam emission surface 101. The thermal electrons emitted from the electron beam emission surface 101 collide with a positive-electrode target (not shown in the figures) to generate an X-ray.
Of the legs 102 to 105, the legs 102, 103 (indicated by “A” and “B” in the figures) are legs 102, 103 for full lighting, electrical heating, the legs 102, 103 being used for full lighting for large focus. In the full lighting for large focus, power is distributed to heat the entire region of the electron beam emission surface 101 to emit an electron beam. Of the legs 102 to 105, the legs 104, 105 (indicated by “C” and “D” in the figures) are, on the other hand, legs 104, 105 for half lighting, electrical heating, the legs 104, 105 being used for half lighting for small focus. In the half lighting for small focus, power is distributed to heat only a narrower region (see the region hatched using lines obliquely extending toward the upper right side in the figures) than the entire surface of the electron beam emission surface 101 to emit an electron beam.
That is, in the case of heating the entire region of the electron beam emission surface 101, power is distributed through the legs 102, 103 (A, B) for full lighting, electrical heating, thereby heating the entire region of the electron beam emission surface 101. On the other hand, in the case of limiting, for small focus, the electron emission area by partial lighting, power is distributed through the legs 104, 105 (C, D) for half lighting, electrical heating, thereby lighting and heating only the region hatched using the lines obliquely extending toward the upper right side in the figures. In the case of the full lighting, the electric flow path is in the order of A, the base end of A, the base end of D, the base end of C, the base end of B, and B. In the case of the half lighting, the electric flow path is in the order of D, the base end of D, the base end of C, and C. In this manner, the lighting area of the flat plate-shaped filament is adjusted by a change in the electric flow path (see, e.g., Patent Document 1).